1. Field of the Invention
The invention relates to a length-adjustable column for chairs or the like, comprising an upright tube with a longitudinal axis, and a piston-and-cylinder unit received in the upright tube. A first lengthwise end of the piston-and-cylinder unit being radially supported in the upright tube and displaceably guided in the direction of the longitudinal axis, while a second lengthwise end of the piston-and-cylinder unit is locked in the axial direction by means of an axial bearing on a base plate of the upright tube having an aperture. The bearing is on the one hand fixedly connected in the axial direction to the second lengthwise end of the piston-and-cylinder unit, and on the other hand, formed with a latching arrangement which fixes the axial bearing in the axial direction on the upright tube by engagement into the aperture of its base plate.
2. Description of the Related Art
The axial bearings of such length-adjustable columns have, as indicated above, the function of fixedly connecting the second lengthwise end of the piston-and-cylinder unit to the upright tube in the direction of the longitudinal axis of the upright tube. However, it is generally desirable, especially in length-adjustable columns for swivel chairs, for the piston-and-cylinder unit to be able to rotate about the longitudinal axis of the upright tube despite being fixed in the axial direction in the upright tube. Customarily, this possibility of rotation is likewise provided by the axial bearing, or more accurately by its connection to the second lengthwise end of the piston-and-cylinder unit and/or its connection to the base plate of the upright tube.
U.S. Pat. No. 6,007,044 discloses a generic, length-adjustable column with such an axial bearing. The axial bearing, formed as an injection molding, has a fixing pin at its lower end which is intended to engage into the aperture in the base of the upright tube. The outer diameter of this fixing pin is, at its widest point, less than the inner diameter of the aperture in the base plate. A spreader ring with a frustoconical outer circumferential surface is inserted into an annular groove in the fixing pin. In its relaxed state, the spreader ring has an outer diameter that is greater than the inner diameter of the aperure of the base of the upright tube. However, when the fixing pin is introduced into that aperture, the spreader ring can be compressed to a diameter which is less than the inner diameter of the aperture of the base plate of the upright tube. If the fixing pin has passed completely through the aperture, the spreader ring expands again as a result of its intrinsic elasticity, engages behind the aperture and thus secures the axial bearing on the base plate. A disadvantage of this embodiment is that the fixing pin and the spreader ring have to be produced to close tolerances in order to be able to ensure firm seating of the axial bearing on the base plate. As a result of these narrow tolerances, however, it may occur during installation of the axial bearing that the spreader ring, after the introduction of the fixing pin into the base plate aperture, does not re-expand to the necessary extent because of excessive friction. As a result, in the event of the exertion of upward-directed axial forces away from the base plate, the axial bearing may again becomes detached from the base plate.
U.S. Pat No. 5,120,011 discloses an axial bearing, formed as a rolling contact bearing, with a cage which passes through the base of the upright tube and is locked to the piston rod. DE 21 64 196 C3 discloses a sliding bearing which is elastically locked to the piston rod. U.S. Pat. No. 4,729,458 discloses securing the locking of the piston-and-cylinder unit on the base of the upright tube by means of a separate pin introduced from below.
Additional prior art that may be mentioned for the sake of completeness includes GB 1 239 729 A, EP 0 477 617 B1, EP 0 366 889 B1, DE 1 961 656, DE-GM 67 53 661, DE-C 1 779 813 and FR 2 022 207 A.
By comparison with the above, it is an object of the invention to provide a length-adjustable column of the type mentioned initially which offers increased security against unintentional releasing of the axial bearing from the base plate of the upright tube of the column.
According to the invention, the latching arrangement has at least two locking arms and a securing bolt, the securing bolt being insertable between the locking arms in the course of production of the axially fixed connection between the axial bearing and the second lengthwise end of the piston-and-cylinder unit from this second lengthwise end of the piston-and-cylinder unit and thus securing the locking of the latching arrangement in the aperture of the base plate of the upright tube. In its position inserted between the locking arms, the securing bolt prevents the arms from moving toward one another, for example under the influence of external forces, and thus ensures that the axial bearing remains securely locked in the aperture of the base plate.
The securing bolt can be integrally molded on the axial bearing, preferably on a base disk of the axial bearing intended to rest on the base plate of the upright tube. This embodiment has the advantage that, in order to provide the complete axial bearing including all parts of the latching arrangement, only a single part needs to be manufactured. This part combines all the functions required of the axial bearing including the securing of the latching arrangement against unintentional release from the base of the upright tube. As a result of this design, moreover, preassembly steps for the axial bearing, as required for example in the axial bearing known from U.S. Pat. No. 6,007,044 in which the spreader ring has to be plug-fitted onto the fixing pin in a preassembly step, become unnecessary.
In order to enable the securing bolt to be inserted between the locking arms, in a further development of this embodiment, the securing bolt can be connected to the axial bearing, preferably the base disk thereof, via at least one predetermined breaking point. The predetermined breaking point here can be formed by a web of material connecting the securing bolt to the axial bearing, preferably the base disk thereof, and/or by a strip of material extending over at least part of the circumference of the securing bolt and connecting the securing bolt to the axial bearing, preferably the base disk thereof.
In order to prevent the securing bolt obstructing the insertion of the latching arrangement into the aperture in the base of the upright tube, it is further proposed that the securing bolt be arranged, in a ready-to-fit state of the axial bearing, substantially completely outside the area between the locking arms.
The locking arms may be designed in various ways: for example, at least one of the locking arms can be elastically lockable to the base plate of the upright tube. In addition, or alternatively, it is also possible for the locking arms to be spread as a result of the insertion of the securing bolt.
In order to be able to prevent unintentional dropping of the securing bolt out of the area between the locking arms, the latching arrangement may have at least one axial stop for the securing bolt. This at least one axial stop may, for example, be formed by at least one shoulder on at least one of the locking arms. In addition, or alternatively, the securing bolt may have at least one lengthwise groove which is open at its end nearer the locking arm and closed at its end further from the locking arm in the direction of the longitudinal axis, and the axial bearing has at least one shoulder which engages into an assigned lengthwise groove.
For axially fixing the axial bearing on the second lengthwise end of the piston-and-cylinder unit, the axial bearing, preferably the base disk thereof, can be provided in a conventional manner with a snap-engagement arrangement. According to a structurally simple embodiment, this snap-engagement arrangement may have at least one snapping arm. To retain the second lengthwise end of the piston-and-cylinder unit, this at least one snapping arm may have a locking surface which is, for example, substantially trapezoidal in section and intended to engage into a locking recess provided on the circumference of the second lengthwise end of the piston-and-cylinder unit. In order to be able to provide a connection between the axial bearing and the piston-and-cylinder unit in the direction of the axis of the upright tube which is firm but allows twisting about that axis, the locking recess on the circumference of the second lengthwise end of the piston-and-cylinder unit can be a locking groove extending over the entire circumference.
To facilitate installation of the piston-and-cylinder unit into the upright tube, it is also proposed that the axial bearing comprise a cage to receive, preferably with a friction fit, the second lengthwise end of the piston-and-cylinder unit. This is because the cage makes it possible for the axial bearing to be plug-fitted onto the second lengthwise end of the piston-and-cylinder unit and introduced into the upright tube together with the latter. Thus, no elaborate preassembly of the axial bearing in the aperture of the base of the upright tube is necessary. Instead, the axial bearing can be installed together with the piston-and-cylinder unit.
The cage may, for example, have a holding ring which is connected to a base disk of the axial bearing via at least one connecting web. At least one elongate projection extending in the axial direction can be provided on an inner surface of the holding ring and/or of the at least one connecting web. This at least one projection serves to rest on the second lengthwise end of the piston-and-cylinder unit and, because of its relatively small contact surface with the latter, prevents the occurrence of excessive frictional forces between piston-and-cylinder unit and axial bearing even when production tolerances are generous.
In principle, it is possible for the at least one above-mentioned snapping arm of the snap-engagement arrangement to start from the base disk of the axial bearing. According to the invention, however, the at least one snapping arm extends from the holding ring toward the base disk.
As is known per se, the second lengthwise end of the piston-and-cylinder unit can be formed by the free end of a piston rod of this piston-and-cylinder unit. In order to be able to prevent the piston rod breaking through the base of the upright tube in the serious event of destruction of the axial bearing, it is proposed that the piston rod have a greater diameter than the aperture in the base of the upright tube.
From another standpoint, the invention relates to an axial bearing having the axial bearing features explained above.
Finally, the invention also relates to a method for installing a piston-and-cylinder unit in an upright tube of a length-adjustable column for chairs or the like, using an axial bearing according to the invention. The installation method according to the invention comprises the following steps:
plug-fitting the axial bearing with its cage onto the second lengthwise end of the piston-and-cylinder unit,
introducing the piston-and-cylinder unit preassembled in this way into the upright tube with the second lengthwise end foremost,
exerting a first axial installing force on the piston-and-cylinder unit, which is sufficient to introduce the latching arrangement sufficiently far into the aperture in the base plate of the upright tube for the axial bearing to rest on the base plate, and
exerting a second axial installing force on the piston-and-cylinder unit which is greater than the first installing force and which is sufficient to introduce the securing bolt between the locking arms of the latching arrangement.
Advantageously, the second axial installation force can be such as to suffice to destroy the predetermined breaking points by means of which the securing bolt is retained on the axial bearing.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.